132 



KNOWLEDGE. 



[June 1, 1898. 



very attentive way m which all our needs were provided 

 for by the Deputy Commissioner of the district, Lieutenant 

 Morris, I was able to get everything erected and in 

 working order during the fortnight preceding the ecHpse. 

 In adjusting the instruments and putting together the six- 

 inch telescope and spectrograph I had also the advantage 

 of receiving moat elBcient help from Captain Molesworth, 

 R.E., without whose skilled assistance it would have been 

 impossible to get all ready in time. 



On the day of the eclipse the actual procedure was as 

 follows : — About ten minutes before totality the heliostat 

 was started going and the mirrors adjusted. Then the 

 exposing cap of the prismatic camera was put on and the 

 first plate drawn up into position by means of a rack- 

 and-pinion arrangement which I had made for this instru- 

 ment to obviate the necessity for " changing plates " 

 after each exposure. Next, the exposing shutter of the 

 slit spectrograph was closed and the dark slide drawn 

 out ready. 



At eighty-eight seconds before second contact the six- 

 inch telescope was adjusted and clamped in such a position 

 that the diurnal motion would carry the image of the 

 eastern edge of the sun exactly into the middle of the 

 field of the spectrograph at the moment of second contact. 

 This was effected by moving the telescope until the image 

 of what remained of the sun touched a certain mark 

 previously made on a screen placed in the focal plane, 

 and keeping it there by following in R.A. until the 

 chronometer I was using indicated eighty-eight seconds 

 before totality. 



During the last half-minute before the eclipse was total 

 I began exposures with the prismatic camera, taking two 

 instantaneous photographs of the cusp spectrum, and then 

 drawing another plate into position ready for the " flash." 



Now, all being ready, only a few seconds remained 

 before the bright lines of the flash spectrum might be 

 expected to appear. The gloom of the approaching 

 shadow was already increasing at an alarming rate. I 

 turned to the visual spectroscope, took oS' the slit head, 

 and watched the spectrum of the last remaining thread of 

 sunlight without any slit. The well-known j,'roups of dark 

 lines composing the ordinary solar spectrum were seen at 

 first just as though the slit had not been removed, but 

 they were curved arcs instead of straight lines, each taking 

 the form of the little crescent of photosphere remaining 

 uncovered. 



The band of continuous spectrum in which these dark 

 lines appeared was seen to be rapidly narrowing, but, 

 instead of thinning down to a single thread, the roughness 

 of the moon's edge caused it to suddenly break up into a 

 number of strips with dark spaces between, and at this 

 instant the bright lines flashed out in hundreds between 

 and across the streaks of continuous spectrum. I was 

 astonished at the suddenness of the reversal from dark 

 lines to bright, and at the brilliancy and extreme sharpness 

 of the lines ; many of them extended for thirty degrees or 

 more round the limb of the moon, but interrupted here 

 and there by the projecting lunar mountains. 



Without waiting for further developments I immediately 

 exposed the prismatic camera and the large spectrograph, 

 in hopes that the photographic plate would be equal to the 

 occasion and duly record this wonderful display. 



In the resulting photograph (No. 3) certainly not all of 

 the finer lines are depicted which I could see reversed in 

 the part of the spectrum I was observing. But in the 

 ultra-violet, where the definition is best, an extraordinary 

 wealth of fine lines are shown ; and this end of the photo- 

 graph gives a good idea of what I actually saw near the 

 group /' in the green. 



Immediately after second contact I made an instan- 

 taneous exposure with the prismatic camera — the fourth of 

 the series — and then started a series of long exposures, at 

 the same time opening the shutter of the slit spectrograph. 



During the first long exposure I left the seat near the 

 heliostat and closed the slide of the large spectrograph, 

 reversed it, and opened again ready for the second flash. 

 Then I had to turn the right ascension handle four revolu- 

 tions to bring the west limb into the field of the spectro- 

 graph at third contact. The forty seconds occupied in 

 this way with my back to the eclipse was an ordeal which 

 I trust I may never again have to undergo ! After returning 

 to the seat I closed the long exposure and started another ; 

 then I had a look, for the first time, at the corona. With 

 a pair of binoculars I examined the beautiful streamer on 

 the south-west side which was so successfully photographed 

 by Mrs. Maunder (see the May Number of Knoi\xedge). 

 But almost before I could gain any very distinct impressions 

 I was interrupted by the time caller, only twenty more 

 seconds remaining before the sun would reappear ! It was 

 necessary to close the long exposure, expose another short 

 one, and then look out for the flash again. 



Fortunately I made the exposure for the second flash 

 spectrum just as the first points of sunlight burst into 

 view on the west limb, forming what is known as 

 " Bailey's Beads." (See No. 8.) 



Two more snap shots with the prismatic camera taken 

 in rapid succession completed the programme. 



The whole performance seemed to have gone ofif without 

 any serious hitch, but too late I discovered the slit spectro- 

 graph still open, with the crescent sun right across the 

 slit ! I had forgotten to close the shutter in the hurry of 

 the last moments of totality. 



The number of photographs secured altogether was 

 thirteen ; one with the slit spectrograph, two with the 

 large spectrograph, and ten with the prismatic camera. 



The single photograph obtained with the first named 

 failed from the above-mentioned cause, the direct sunlight 

 and halation nearly obliterating the faint coronal spectrum. 



The large spectrograph yielded two negatives of the 

 flash spectrum which show a considerable number of lines 

 in a limited region of the spectrum, but on the whole they 

 do not quite come up to expectation. The best results 

 were those obtained with the prismatic camera. This 

 instrument gave images of the spectrum extending from 

 A 600 in the orange to A 338 in the ultra-violet. The 

 scale of the original negatives is -33 inch to the moon's 

 diameter and one inch from F to H, the total length of 

 spectrum photographed being 2-8 inches. 



All the ten exposures yielded good negatives. Nos. 1, 2, 

 9, and 10 of the series give the spectrum of the cusps just 

 before and just after the total phase. They show the 

 P^aunhofer dark-line spectrum bordered with bright lines, 

 and in \o. 10 nil the dark lines in the ultra-violet end in 

 a short bright line. 



The flash spectrum lines are shown in Nos. 3, 7, and 8. 

 In No. 3 they are beautifully defined in the ultra-violet 

 from H upwards, and this photograph is certainly the 

 finest of the set. It will be necessary to study it in great 

 detail by means of enlargements and with the help of the 

 Fraunhofer spectrum (obtained under precisely the same 

 angle of incidence) given in Nos. 1 and "2. 



Of the remaining plates, No. 4 reveals a very curious 

 feature in the prominence spectrum. In the ultra-violet 

 are seen a succession of little dots due to the hydrogen 

 radiations, and at the point where these terminate (at 

 K 3660) the spectrum abruptly changes its character and 

 becomes a continuous one, a delicate line running from 

 the last dot to the end of the plate. 



